From the printed tube curves, you can check your tester at any operating point you need. This works as follows: Selecting the anode voltage needed, and go vertical, to cross two tube curves, which are the working points and mark the cross point with a dot. From each dot (1 and 2) , write down: Ug1, Ia1, and Ug2, Ia2. WIth these numbers, calculate: Gm= (Ia2-Ia1)/(Ug2-Ug1), and if needed: Ra=(Ua1-Ua2) / (Ia2-Ia1). Then, gain = Gm * Ra. The printed test report has this already done, for the standard operating point of the tubes. This is 250V/73mA for the 6L6, 250V/9mA for the 6SN7, and 250V/1.2mA for the 6SL7.